Fix #106 using new Cesium Bing Maps key, other Assets/

Cesium's Bing Maps key expired so their new code gives us a fresh
one.

Might as well use all their Assets as well, so we don't have to
maintain our own SkyBox and static Earth map.  Update the docs to copy
their CesiumUnminified/ directory into our tree as cesium/, then
remove our unnecessary copies.